Since surfaces of materials such as silicon, hard disks, mother glass for liquid crystal displays, and semiconductor devices need to be flat, the surfaces are polished by the loose abrasive method using a polishing pad. The loose abrasive method is a method in which the work surface of a workpiece is polished while a slurry (polishing liquid) containing abrasive grains is supplied between a polishing pad and the workpiece.
In a polishing pad for a semiconductor device, the surface of the polishing pad is required to have openings for retaining abrasive grains, rigidity for keeping the flatness of the surface of the semiconductor device, and elasticity for preventing scratches on the surface of the semiconductor device. As a polishing pad meeting these requirements, a polishing pad having a polishing layer manufactured from a urethane resin foam has been used.
In general, a urethane resin foam is molded by curing a prepolymer containing an isocyanate group-containing compound based on a reaction with a curing agent (dry method). Then, a polishing pad is formed by slicing this foam into a sheet shape. In a polishing pad (hereinafter, abbreviated as hard (dry) polishing pad in some cases) having a hard polishing layer molded by the dry method as described above, relatively small and substantially spherical cells are formed inside the foam during the curing and molding of the urethane resin. Hence, openings (pores) capable of retaining a slurry during polishing are formed on the polishing surface of the polishing pad formed by the slicing.
Urethane resin foams having cell (air-bubble) diameters of 100 μm or less and around 30 μm have been mainly used as materials of polishing pads for a semiconductor device so far (Patent Literature 1). In addition, the mainly used urethane resin foams have type A hardnesses of 70 degrees or higher, and type D hardnesses of 45 or higher (Patent Literatures 2 and 3). The mainly used urethane resin foams have densities of 0.5 g/cm3 or higher (Patent Literature 1) and, regarding the elasticity, have storage elastic moduli of several hundred megapascals or higher (Patent Literature 4). A mainly employed longitudinal elastic modulus (Young's modulus) is 500 MPa or higher (Patent Literature 5).
Apart from the above-described mainly used urethane resin foams, physical properties of urethane resin foams have been improved from the viewpoints of the bulk density, the type A hardness, and the hard segment content (HSC) (%) in order to reduce wearing to an appropriate degree and stabilize the polishing performance (Patent Literature 6). Moreover, polishing pads are also reported whose storage elastic moduli are adjusted to be within predetermined ranges in order to reduce the occurrence of scratches (Patent Literatures 7 and 8).